The present invention relates to electronic data communications. More particularly, the disclosure relates to timing jitter and compensating for timing jitter in a high speed communication link.
Signal integrity issues such as timing jitter are at the forefront of high-speed digital design for communication applications. Electronic circuit speeds are overwhelming the legacy channels that traditionally could be treated as ideal. In high-speed data circuits, the channel behavior is typically compensated appropriately to enable the highest information capacity. Noise considerations dictate the choice of equalization technique.
In serial communications, data encoded as symbols are transmitted over a channel such as optical fiber or copper backplane. Physical characteristics of the channel can dictate an optimal modulation scheme and appropriate symbols. To utilize the channel efficiently, the highest symbol rate is desired. However, if the symbol rate exceeds the bandwidth of any component in the communication link, errors often occur during the data transmission. Insufficient bandwidth does not allow the signal to make a complete transition within a symbol period and the margin between symbol levels is reduced. This well-known penalty is intersymbol interference (ISI). Furthermore, insufficient bandwidth causes the symbol timing at the receiver to deviate from the timing at the transmitter. The total deviation, called timing jitter, is exacerbated in high-speed communication systems and jitter requirements are increasingly restrictive.
Jitter is deviations in the timing of received data bits compared to a reference, such as a data stream in the transmitter. The quality of a received data stream can be analyzed by examining a data eye that is generated by overlaying the received data stream over a time period sufficient to allow all possible data states and state transitions to occur. Data jitter reduces the horizontal opening of the data eye.
Timing jitter is composed of random and deterministic terms that quantify the total jitter. Random jitter is typically a Gaussian distribution with variance related to the transition characteristic of the system. Two forms of deterministic jitter (DJ) are data-dependent jitter and duty-cycle distortion. Data-dependent jitter (DDJ) refers to the impact of the previous symbols on the current timing deviation. Common sources of DDJ include finite system bandwidth and signal reflection. Duty-cycle distortion (DCD) results from the asymmetric response characterized with different rise and fall times of data signal transitions. DDJ and DCD tend to dominate in serial links.
A simple communication link introduces several sources of jitter. Generally, all components within a link are typically designed to meet a jitter budget since the jitter accumulates. Jitter generated in the transmitter increases through any regenerating stage in the link. Furthermore, the signal is attenuated during transmission over the channel, reducing the signal-to-noise ratio and limiting the sensitivity of the receiver. The receiver has amplification stages with a given bandwidth to limit the noise. The timing jitter is often most severe after this amplification at the input to the clock and data recovery (CDR) circuit.
In high-speed data circuits the sampling clock is typically recovered from the edges of received data to eliminate the need to separately communicate a clock signal. Therefore, the data jitter deviations translate to phase noise in the recovered clock and consequently sampling uncertainty in the data eye. This uncertainty reduces the receiver bit error rate (BER) performance.
BER requirements compel limiting the jitter from the standpoint of decision errors and the performance of the CDR circuit. Additionally, managing jitter can loosen the restrictions on the jitter transfer and, hence, the bandwidth of the clock recovery, reducing acquisition time of the CDR circuit.
The symbol is typically detected in the CDR circuit. The data recovery circuit samples the corrupted data in the time domain with a local oscillator. Horizontal eye closure due to DJ reduces the range of times that accurately sample the data. Additionally, the local oscillator is synchronized to the data transitions. Therefore, timing jitter of the data disturbs the oscillator and the sampling uncertainty is increased. Accordingly, DJ has time and frequency domain interpretations.
From the above, it is seen that techniques for improving data communications are highly desired.